void merge_sort(int q[], int l, int r){
    if (l >= r) return;

    int mid = l + r >> 1;
    merge_sort(q, l, mid);
    merge_sort(q, mid + 1, r);

    int k = 0, i = l, j = mid + 1;
    while (i <= mid && j <= r){
        if (q[i] <= q[j])
            tmp[k++] = q[i++];
        else
            tmp[k++] = q[j++];
    }
    while (i <= mid) tmp[k++] = q[i++];
    while (j <= r) tmp[k++] = q[j++];

    for (i=l, j=0; i<=r; i++,j++)
        q[i] = tmp[j];
}